The propagation delays between a network base station and individual wireless communication devices transmitting uplink signals to the base station depends on the respective distances between the individual devices and the base station. Timing Advance or “TA” techniques provide for an adjustable delay at each wireless communication device, which controls the timing at the device between the start of a received downlink subframe and a transmitted uplink subframe. By dynamically adjusting the TA values used by the various devices, the base station ensures that uplink signals from the various devices are time aligned at the base station. In turn, receiving time-aligned uplink signals from multiple devices preserves the orthogonality between those uplink signals, as received at the base station.
In an example configuration, smaller TA values represent less timing advance and larger TA values represent more timing advance. In turn, more timing advance means less delay at the device between the start of a received downlink subframe and the corresponding uplink subframe transmission. Thus, devices that are further away from the base station and have longer signal propagation delays with respect to the base station use larger TA values, and devices that are closer to the base station use smaller TA values.
In a known approach, a base station determines the appropriate TA value for any given device based on measurements of its uplink signals. In networks based on the Long Term Evolution, “LTE”, standard, a User Equipment, “UE”, obtains initial uplink synchronization in a given cell using a random access procedure. Here, the term “cell” connotes given air interface resources within a given geographic coverage area. Thus, two cells may wholly or partly overlap geographically but use different carrier frequencies or different frequency subbands within a defined carrier frequency bandwidth, for example.
In any case, according to known TA value initialization procedures used to gain initial uplink synchronization with a given cell in the network, the UE transmits a preamble on a Random Access Channel, “RACH”. The cell's eNodeB—an LTE base station—determines an initial TA value for the UE based on measurements performed on the preamble transmission, and the eNodeB transmits the initial TA value to the UE in a random access response message. The initial TA value is an absolute TA value of 11 bits. The eNodeB subsequently adjusts the UE's TA value, as needed, to maintain the UE in uplink synchronization with the cell. The subsequent adjustments are based on the eNodeB sending Timing Advance Command Medium Access Control, “MAC”, Control Elements or “CEs”. These subsequent TA commands are 6 bits and represent delta updates to the TA value.
The same TA value may be applied by the UE for its uplink transmissions to a group of cells in the network, where such groups are referred to as Timing Advance Groups or “TAGs”. Each TA group has one TA value and an associated TA timer. This arrangement complicates evolving service scenarios, such as those based on Carrier Aggregation, “CA” and/or Coordinated Multi-Point, “CoMP” service.
With CA, more than one carrier is used to serve a UE or other wireless communication device. These multiple carriers are referred to as Component Carriers or “CCs” and they generally include a Primary Carrier from a Primary Cell or “PCell” and at least one Secondary Carrier from a Secondary Cell or “SCell”. Among the several cells involved in CA service, the Primary Carrier from the PCell serves as a reference or anchor carrier that is used by the UE for radio link failure monitoring and certain other reference functions.
CoMP expands the multiple carrier idea by using a coordinated set of eNodeBs and/or other transmission/reception points in the network to serve a given UE. In general, at any given time, only a subset of cells within a CoMP cluster is used to serve a given UE. However, that subset dynamically changes as given cells in the CoMP cluster become more or less attractive for use in serving the UE, based on cell loading, changes in the location of the UE relative to the various CoMP transmission/reception points in the cluster, and other factors.
According to recent agreement in the Third Generation Partnership Project, “3GPP”, for serving cells that are in the same TAG as the UE's PCell, the downlink reception timing of the PCell serves as the timing reference. For serving cells in a TAG not containing the UE's PCell, the downlink reception timing of a serving cell selected by the UE should be used as the downlink timing reference. Also, note that when a UE receives an initial or subsequent TA command, it starts the TA timer associated with the TAG for which the TA command was received. The UE considers itself to be in uplink synchronization with the cells belonging to the TAG associated with the received TA command so long as the associated TA timer is running Thus, the UE may perform Physical Uplink Shared Channel, “PUSCH”, and Physical Uplink Control Channel, “PUCCH”, transmissions in those cells.
However, as noted above, if the UE loses uplink synchronization with the cell(s) in a TAG, or wishes to transmit on cell(s) in a new TAG with which it has not gained uplink synchronization, the UE must carry out the aforementioned random access procedure and then use the initial TA value returned from the network before performing any PUSCH or PUCCH transmissions on the cell(s) in the TAG. Such requirements complicate operation, particularly in scenarios like CoMP, in which a dynamically changing mix of cells is used to serve a given UE. Moreover, it is recognized herein that such requirements result in significant inefficiencies in many practical scenarios involving the creation of new TAGs for a UE or other device operating within a wireless network.